Secondary batteries have attracted considerable attention as energy sources for wireless mobile devices. In addition, secondary batteries have attracted considerable attention as power sources for electric vehicles (EV), hybrid electric vehicles (HEV), and plug-in hybrid electric vehicles (Plug-in HEY), which have been developed to solve problems, such as air pollution, caused by existing gasoline and diesel vehicles using fossil fuels.
Such electric vehicles, hybrid electric vehicles and plug-in hybrid electric vehicles are devices which must be operated under more harsh conditions than small-sized mobile devices. That is, it is necessary for electric vehicles, hybrid electric vehicles and plug-in hybrid electric vehicles to exhibit proper performance in summer, for example at high temperature, and in winter, for example at low temperature.
To obtain good high temperature performance, a secondary battery having high temperature safety has been manufactured or a method of forming a coolant channel in a battery module or a battery pack has been used.
At low temperature, on the other hand, the secondary battery exhibits lower efficiency than at high temperature because the internal resistance of the battery is high at low temperature. As a result, the capacity of the battery is restricted and the liftspan of the battery is reduced.
In order to solve the above problems, a method of improving the low temperature performance of a battery has been studied. In such a battery, however, the low temperature performance of the battery is improved but the capacity and the high temperature performance of the battery are deteriorated.
As another method of improving low temperature performance of the battery, a method of increasing the temperature of the battery using an additional heating apparatus (heater) may be considered. However, much time is taken due to high thermal capacity of the battery, and the capacity of the battery is reduced due to internal power consumption.
Consequently, there is a high necessity for a battery pack system that is capable of minimizing internal power consumption while exhibiting excellent performance at low temperature without change in battery performance.